Problem 13 - Chemistry The Central Science

Question

(a) What is the difference between the Arrhenius and the Bronsted-Lowry definitions of an acid? (b) \(\mathrm{NH}_{3}(g)\) and \(\mathrm{HCl}(g)\) react to form the ionic solid \(\mathrm{NH}_{4} \mathrm{Cl}(s) .\) Which substance is the Bronsted-Lowry acid in this reaction? Which is the Bronsted-Lowry base?

Step-by-Step Solution

Answer

(a) The Arrhenius definition states that an acid is a substance that dissociates in water to produce hydrogen ions (H+), while a base produces hydroxide ions (OH-). The Bronsted-Lowry definition states that an acid is a proton (H+) donor and a base is a proton (H+) acceptor. (b) In the reaction \(\mathrm{NH}_{3}(g) + \mathrm{HCl}(g) \rightarrow \mathrm{NH}_{4} \mathrm{Cl}(s)\), HCl is the Bronsted-Lowry acid as it donates a proton to NH3. NH3 is the Bronsted-Lowry base since it accepts the proton from HCl.

Step 1: Define the Arrhenius and Bronsted-Lowry acids

According to the Arrhenius theory, an acid is a substance that dissociates in water to produce hydrogen ions (H+) while a base is a substance that dissociates in water to produce hydroxide ions (OH-). Meanwhile, the Bronsted-Lowry theory states that an acid is a proton (H+) donor, and a base is a proton (H+) acceptor.

Step 2: Identify the Bronsted-Lowry acid and base in the given reaction

We have the reaction: \(\mathrm{NH}_{3}(g) + \mathrm{HCl}(g) \rightarrow \mathrm{NH}_{4} \mathrm{Cl} (s)\) In this reaction, the NH3 molecule accepts a proton (H+) from the HCl molecule to form the NH4+ ion, while the Cl- ion is left behind. According to the Bronsted-Lowry definition, the proton donor is the acid and the proton acceptor is the base.

Step 3: Determine the Bronsted-Lowry acid and base

In the reaction: \(\mathrm{NH}_{3}(g) + \mathrm{HCl}(g) \rightarrow \mathrm{NH}_{4} \mathrm{Cl}(s)\) The HCl molecule donates a proton (H+) to the NH3 molecule, making it the Bronsted-Lowry acid. The NH3 molecule accepts the proton from HCl, making it the Bronsted-Lowry base.

Key Concepts

Arrhenius Theory

The Arrhenius Theory is one of the earliest models to define acids and bases. According to Svante Arrhenius, an acid is a substance that increases the concentration of hydrogen ions ( \(\text{H}^+\)) in aqueous solution. This happens when the substance dissociates in water:

• Example: Hydrochloric acid ( \(\text{HCl}\)) dissociates to produce \(\text{H}^+\) and \(\text{Cl}^-\).

On the other hand, a base under the Arrhenius definition produces hydroxide ions ( \(\text{OH}^-\)) in water.

• Example: Sodium hydroxide ( \(\text{NaOH}\)) dissociates to produce \(\text{Na}^+\) and \(\text{OH}^-\).

This theory is simple but limited because it only considers reactions in aqueous solutions and doesn't account for reactions involving proton transfer without water.

Bronsted-Lowry Acid

The Bronsted-Lowry theory expands the definition of an acid beyond what Arrhenius described. Proposed by Johannes Nicolaus Bronsted and Thomas Martin Lowry, an acid is defined as a proton donor in this model. This means any molecule or ion that can donate a hydrogen ion ( \(\text{H}^+\)) is considered an acid.

• Example: When \(\text{HCl}\) reacts with water, it donates a proton to form \(\text{H}_3\text{O}^+\) and \(\text{Cl}^-\).

Such a definition allows the concept of acids to be applied in non-aqueous environments, making it more versatile for various chemical reactions.

Bronsted-Lowry Base

Complementary to the Bronsted-Lowry definition of an acid, a base is defined as a proton acceptor. In this sense, a base is any species capable of accepting a hydrogen ion ( \(\text{H}^+\)).

• In the reaction \(\text{NH}_3(g) + \text{HCl}(g) \rightarrow \text{NH}_4\text{Cl}(s)\), ammonia ( \(\text{NH}_3\)) acts as a base by accepting a proton from hydrochloric acid ( \(\text{HCl}\)).

This broadened definition is critical for understanding reactions that occur outside of water, demonstrating the flexibility of the Bronsted-Lowry theory.

Proton Transfer Reactions

Proton transfer reactions are fundamental to the Bronsted-Lowry theory of acids and bases. These reactions involve the movement of a proton ( \(\text{H}^+\)) from an acid (proton donor) to a base (proton acceptor).

• For instance, in the reaction \(\text{NH}_3(g) + \text{HCl}(g) \rightarrow \text{NH}_4\text{Cl}(s)\), a proton is transferred from \(\text{HCl}\) to \(\text{NH}_3\)

Understanding proton transfer is essential, as it highlights the reactive nature of these substances and the formation of new products. This is crucial for balancing chemical equations and predicting the outcome of reactions.